I do wall sits, they say it's one of the safest excersizes you can do, and lift 5 lb weights to keep my arms toned. But walking is definitely a good starting point.
Walking is always a safe starting point unless you're on bed rest! I walk and jog a bit. I toss a football around the yard with my hubby. I'll be swimming once it's a bit warmer. Talk to your doctor before trying anything you're not already used to. And no flat on your back exercises.